Characterization of aluminum-doped zinc oxide thin films by RF magnetron sputtering at different substrate temperature and sputtering power

In this study, transparent conductive Al doped zinc oxide (ZnO: Al, AZO) thin films with a thickness of 40 nm were prepared on the Corning glass substrate by radio frequency magnetron sputtering. The properties of the AZO thin films are investigated at different substrate temperatures (from 27 to 150 °C) and sputtering power (from 150 to 250 W). The structural, optical and electrical properties of the AZO thin films were investigated. The optical transmittance of about 78 % (at 415 nm)–92.5 % (at 630 nm) in the visible range and the electrical resistivity of 7 × 10^?4 Ω-cm (175.2 Ω/sq) were obtained at sputtering power of 250 W and substrate temperature of 70 °C. The observed property of the AZO thin films is suitable for transparent conductive electrode applications.     

Quality enhancement of AZO thin films at various thicknesses by introducing ITO buffer layer

Due to the simultaneously superior optical transmittance and low electrical resistivity, transparent conductive electrodes play a significant role in semiconductor electronics. To enhance the electrical properties of these films, one approach is thickness increment which degrades the optical properties. However, a preferred way to optimize both electrical and optical properties of these layers is to introduce a buffer layer. In this work, the effects of buffer layer and film thickness on the structural, electrical, optical and morphological properties of AZO thin films are investigated. Al-doped zinc oxide (AZO) is prepared at various thicknesses of 100 to 300 nm on the bare and 100 nm-thick indium tin oxide (ITO) coated glass substrates by radio frequency sputtering. Results demonstrate that by introducing ITO as a buffer layer, the average values of sheet resistance and strain within the film are decreased (about 76 and 3.3 times lower than films deposited on bare glasses), respectively. Furthermore, the average transmittance of ITO/AZO bilayer is improved nearly 10% regarding single AZO thin film. This indicates that bilayer thin films show better physical properties rather than conventional monolayer thin films. As the AZO film thickness increases, the interplanar spacing, d₍₀₀₂₎, strain within the film and compressive stress of the film in the hexagonal lattice, decreases indicating the higher yield of AZO crystal. Moreover, with the growth in film thickness, carrier concentration and optical band gap (E_g) of AZO film are increased from 4.62?×?10¹⁹ to 8.21?×?10¹⁹ cm^?3 and from 3.55 to 3.62 eV, respectively due to the Burstein-Moss (BM) effect. The refractive index of AZO thin film is obtained in the range of 2.24–2.26. With the presence of ITO buffer layer, the AZO thin film exhibits a resistivity as low as 6?×?10^?4 Ω cm, a sheet resistance of 15 Ω/sq and a high figure of merit (FOM) of 1.19?×?10⁴ (Ω cm)^?1 at a film thickness of 300 nm. As a result, the quality of AZO thin films deposited on ITO buffer layer is found to be superior regarding those grown on a bare glass substrate. This study has been performed over these two substrates because of their significant usage in the organic light emitting diodes and photovoltaic applications as an enhanced carrier injecting electrodes.     

Conductivity enhancement of ZnO:F thin films by the deposition of SnO2:F over layers for optoelectronic applications

Fluorine doped tin oxide (SnO₂:F – FTO) layer is deposited over fluorine doped zinc oxide layer (ZnO:F – FZO) so as to get FTO/FZO bilayers with varying thickness proportions of the two layers using a simplified spray pyrolysis technique. In order to analyse the possible enhancement in the transparent conducting properties of these bilayered films, two separate sets of single layered FTO and FZO films are prepared with similar thickness values and their electrical, optical, photoluminescence and structural characteristics are compared with their bilayered counterparts. The electrical studies revealed that the double-layered films with the lesser thickness of FTO exhibit higher sheet resistance (R_sh) and the R_sh value decreases with the increase in the thickness of FTO over layer. The FTO/FZO bilayer with thickness proportions of FTO:FZO nearly equal to 450 nm:300 nm (3:2) is found to have good figure of merit (quality factor) when compared with FZO films. Even though the optical transmittance (T) in the visible range of FTO/FZO bilayer is lesser (80%) than that of FZO (90%) and FTO (85%) films, the electro-optical properties are reasonably good for the bilayered films making them suitable candidates for opto-electronic applications. The photoluminescence studies support the results obtained in the electrical and structural studies.     

Improved characteristics of sprayed CdO films by rapid thermal annealing

We report the influence of rapid thermal annealing (RTA) on optical, electrical and structural properties of sprayed undoped CdO thin films. XRD investigation revealed that as-grown films are polycrystalline in nature with cubic structure; and pronounced improvement in crystallinity of the films have been noticed after annealing. RTA-treated films showed preferred orientation in the (2 0 0) direction. The optical band gap was deduced from transmittance data for as-grown and annealed films and found to be 2.51 eV for as-grown film and ranged between 2.25 and 2.48 eV for annealed films. Figure of merit was calculated and found its maximum value (1.2 × 10^?2 Ω^?1) was for film annealed with of 6 min. Thermoelectric power (TEP) measurements revealed that the CdO films are having n-type conductivity. Furthermore, the activation energy was calculated for films from TEP data. The lowest electrical resistivity was found to be 6 × 10^?4 Ω cm for CdO film annealed with time ≥6 min. The observed changes demonstrated that RTA is a viable technique for improving characteristics of sprayed CdO films.     

Transparent conductive and infrared reflective AZO/Cu/AZO multilayer film prepared by RF magnetron sputtering

AZO/Cu/AZO multilayer films were prepared on glass substrate by radio frequency magnetron sputtering technology. The prepared films were investigated by a four-point probe system, X-ray diffraction, optical transmittance spectra, scanning electron microscope, atomic force microscopy and Fourier transform infrared spectroscopy. The results showed that Cu inner layer started forming a continuous film at the thickness around 11 nm. The prepared AZO/Cu/AZO samples exhibited the visible transmittance of 60–80 % and sample with 15 nm Cu inner layer showed the highest infrared reflection rate of 67 % in FIR region and the lowest sheet resistance of 16.6 Ω/sq. The proper visible transmittance and infrared reflection property of the AZO/Cu/AZO multilayer film make it a promising candidate for future energy conservation materials.     

Study on the structural, electrical, optical, adhesive properties and stability of Ga-doped ZnO transparent conductive films deposited on polymer substrates at room temperature

Flexible optoelectronic devices are attractive because of light weight, small volume, flexibility and easy transport. Transparent conductive oxide thin films deposited on polymer substrates could satisfy the flexibility for optoelectronic devices. Ga-doped ZnO (GZO) films have been prepared on polycarbonate substrates by radio frequency magnetron sputtering at room temperature. The dependence of the structural, electrical, optical and adhesive properties for films on the sputtering powers was investigated. We also investigated the stability of the electrical property through doing Hall-effect measurements 18 months later. The lowest sheet resistance was 5.8 Ω/sq. After 18 months, the lowest sheet resistance was 6.5 Ω/sq. The stability of the electrical property is excellent. The average transmittance in the visible region of all the films was as high as 85 %, using air as reference. The good transparency-conducting property, excellent stability and room-temperature deposition on polymeric substrates enable GZO films to be widely used in optoelectronic devices.     

Inkjet-printing of antimony-doped tin oxide (ATO) films for transparent conducting electrodes

Antimony-doped Tin oxide (ATO) films have been prepared by inkjet-printing method using ATO nanoparticle inks. The electrical and optical properties of the ATO films were investigated in order to understand the effects of rapid thermal annealing (RTA) temperatures. The decrease in the sheet resistance and resistivity of the inkjet-printed ATO films was observed as the annealing temperature increased. The film annealed at 700 degrees C showed the sheet resistance of 1.7 x 10(3) Omega/sq with the film thickness of 350 nm. The optical transmittance of the films remained constant regardless of their annealing temperatures. In order to further reduce the sheet resistance of the films as well as the annealing temperature, Ag-grid was printed in between two layers of inkjet-printed ATO. With 1.5 mm Ag line spacing, the Ag-grid embedded ATO film showed the sheet resistance of 25.6 Omega/sq after RTA at 300 degrees C.     

High performance humidity sensing properties of indium tin oxide (ITO) thin films by sol–gel spin coating method

The tin doped indium oxide (ITO) thin films prepared by sol–gel spin coating method with In(NO₃)3H₂O and SnCl₄·5H₂O as indium and tin sources respectively is presented. The as deposited samples were annealed at 500 °C for 2 h in order to improve the crystallinity. The structural, morphological and optical properties of the films were analysed by using X-ray diffraction, scanning electron microscope (SEM), UV–Vis transmission spectra and photoluminescence, spectra analysis. The SEM images ensure the uniform and smooth surface of the as prepared and annealed film. The optical transmittance of more than 85 % has been observed in the UV–Vis region with a band gap of 3.91 and 3.73 eV for the as prepared and annealed films of ITO respectively. The PL spectra reveal that the optical properties were significantly improved due to the annealing effect. The annealed film shows high sensitivity for humidity approximately two order changes in the resistance and the sensitivity increases for different relative humidity from 10 to 90 % due to the physisorption between the water molecules and the surface of the thin films.     

The effects of the aluminium concentration on optical and electrical properties of AZO thin films as a transparent conductive layer

In this paper, we studied the effects of the aluminium dopant concentration on the optical and electrical properties of aluminium doped zinc oxide (AZO) thin films grown on soda-glass substrates by a simple chemical method. The amount of aluminium in the compound was varied from 0 to 5 atomic percent (at.%), and the typical thickness of the films produced was about 300 nm. The thin films were characterized by scanning electron microscopy and X-ray diffraction to investigate the morphology and crystallinity of the samples. The optical properties of the thin films were studied by UV–Vis spectroscopy to determinate absorption, transmittance, and the diffuse reflectance. In addition, the photoluminescence properties of the thin films, excited with a 320 nm UV laser beam, were investigated. The effects of the aluminium concentration on these optical properties are discussed. The films with 2 and 5 % doping had excellent optical transmittance (~85–90 %) in the 400–1100 nm wavelength range. The photoluminescence spectra of the AZO films revealed UV near band edge emission peaks in the 378–401 nm range and an oxygen-vacancy related peak around 471 nm. The addition of aluminium changed the band gap of zinc oxide from 3.29 to 3.41 eV, and the appearance of a new level was observed in the band gap at the higher aluminium doping concentrations. The AZO thin films showed good conductivity (in the order of 10^?2 Ω cm) which allows their use as transparent electrodes. Moreover, the AZO thin films were stable in open air for 30 days.     

Effect of the substrate temperature on the properties of spray-deposited SnO2:F thin films

The effect of the substrate temperature on the properties of spray-deposited SnO₂:F thin films is investigated. X-ray diffraction patterns show that the crystallinity of the films is enhanced with the increasing of substrate temperature. Comparing the SEM images, both the particle size and density are increased at a higher deposition temperature. The lowest sheet resistance of 8.43 Ω/□ is obtained at the substrate temperature of 350 °C. In addition, the average optical transmittance of the three films reaches up to 85 % in the visible range. The absorption coefficient is the lowest at 350 °C. The band gap increases from 3.36 to 3.61 eV while the electrical resistivity of SnO₂:F thin films decreases from 8.51 × 10^?3 to 9.86 × 10^?4 Ω cm as elevating the substrate temperature from 250 to 350 °C.     

Electrical and optical properties of ITO:Ca composite thin films for TEOLED cathode

ITO:Ca composite thin films were deposited on glass substrate by the rf magnetron co-sputtering method with various numbers of Ca chips and oxygen partial pressures. The carrier concentration of the ITO:Ca thin film was 7 × 10²⁰ cm^− 3 when the number of Ca chips was 4 at an oxygen partial pressure of 1.4%. The sheet resistance and optical transmittance of the ITO:Ca thin films were 68.2 Ω/sq. and 87%, respectively. The work function of the ITO:Ca thin films with 8 Ca chips was changed from 4.6 eV to 5.0 eV when the oxygen partial pressure was increased from 0.8% to 2.2%. When the oxygen partial pressure was 1.2%, a low work function of 4.6 eV was obtained for the ITO:Ca thin films.     

Effect of SiO2 buffer layer thickness on the properties of ITO/Cu/ITO multilayer films deposited on polyethylene terephthalate substrates

Transparent conductive ITO/Cu/ITO films were deposited on polyethylene terephthalate (PET) substrates with a SiO₂ buffer layer by magnetron sputtering using three cathodes at room temperature. The effect of the SiO₂ buffer layer thickness on the electrical and optical properties of ITO/Cu/ITO films was investigated. The ITO/Cu/ITO film deposited on the 40 nm thick SiO₂ buffer layer exhibits a sheet resistance of 143Ω/sq and transmittance of 65% at 550 nm wavelength. Highly transparent ITO/Cu/ITO films with a transmittance of 80% and a sheet resistance of 98.7Ω/sq have been obtained by applying −60 V substrate bias.     

Effect of SiO2 buffer layer thickness on the properties of ITO/Cu/ITO multilayer films deposited on polyethylene terephthalate substrates

Transparent conductive ITO/Cu/ITO films were deposited on polyethylene terephthalate (PET) substrates with a SiO₂ buffer layer by magnetron sputtering using three cathodes at room temperature. The effect of the SiO₂ buffer layer thickness on the electrical and optical properties of ITO/Cu/ITO films was investigated. The ITO/Cu/ITO film deposited on the 40 nm thick SiO₂ buffer layer exhibits a sheet resistance of 143Ω/sq and transmittance of 65% at 550 nm wavelength. Highly transparent ITO/Cu/ITO films with a transmittance of 80% and a sheet resistance of 98.7Ω/sq have been obtained by applying −60 V substrate bias.     

Structural,morphological, optical and electrical properties of CdS thin films simultaneously doped with magnesium and chlorine

CdS thin films simultaneously doped with Mg and Cl at different doping concentrations (0, 2, 4, 6 and 8 at%) were prepared on glass substrates by spray pyrolysis technique using perfume atomizer at 400 °C. The effect of Mg and Cl doping concentration on the structural, morphological, optical and electrical properties of the deposited films were investigated using X-ray diffraction (XRD), scanning electron microscopy, UV–Vis spectroscopy and dc electrical measurements, respectively. XRD analysis showed that the undoped and doped CdS films exhibit hexagonal structure with a preferential orientation along the (0 0 2) plane. The 2θ angle position of the (0 0 2) peak of the doped films was shifted towards a higher angle with increasing Mg and Cl concentration. The UV–Vis–NIR absorption spectra of Mg and Cl doped thin films are measured and classical Tauc approach was employed to estimate their band gap energies. The increase in band gap energy from 2.46 to 2.73 eV with the reduction in crystallite size supports quantum size effect. Raman spectra implied that more defects existed in the doped samples. Electrical studies showed that all the films have resistivity in the order of 10¹ Ω-cm and the CdS film with 6 at% Mg and Cl concentration has a minimum resistivity of 1.332 × 10¹ Ω-cm.     

Role of solvent volume on the structural and transparent conducting properties of SnO2: Zn films

Zinc doped tin oxide (ZTO) films were deposited from starting solutions having different solvent volumes (10, 20 … 50 mL), using a simplified spray pyrolysis technique. The effects of solvent volume on the structural, electrical, optical and surface morphological properties were investigated. From the structural studies, it is observed that the preferential orientation is along the (200) plane for lower solvent volumes and it turns in favour of (101) plane for the higher solvent volume levels (40 and 50 mL), indicating the stable p-type conductivity. Electrical studies show that the sheet resistance (R_sh) increases as the solvent volume increases. The minimum sheet resistance (4.32 kΩ/sq.) is obtained in the case of minimum solvent volume (10 mL). The Hall measurements reveal that the type inversion takes place beyond 30 mL of solvent volume. The optical transmittance increases as the solvent volume is increased and it is found to be in the range of 80–95 %.     

Structural and plasmonic characteristics of sputtered SnO<Subscript>2</Subscript>:Sb and ZnO:Al thin films as a function of their thickness

Heavily doped metal oxide semiconductors are being developed as thin film transparent electrodes for many applications and their deposition at low substrate temperature can extend the use on heat sensitive devices. The structural and electro-optical characteristics of such metal oxide coatings are tightly related and depend on the specific deposition parameters apart from the material composition. In this work, SnO₂:Sb (ATO) and ZnO:Al (AZO) thin films have been prepared by sputtering at room temperature on glass substrates, changing the deposition time to obtain various layer thicknesses from 0.2 to 0.9 μm; and they have been analyzed by X-ray diffraction, spectrophotometry, and Hall-effect measurements. ATO samples crystallize in the tetragonal structure with mean crystallite size increasing from 8 to 20 nm when the film thickness grows. The comparison of Hall mobility and optical mobility values indicates a significant contribution of grain boundary scattering for these ATO layers. Otherwise, AZO films show larger crystallites (21–27 nm) and a strong preferential orientation for analogous thickness increment, resulting in a lower contribution of the grain boundary scattering to the overall Hall mobility. The in-grain mobility for each sample is also related to the respective crystallite size and carrier concentration values.     

Enhanced excitonic emission and visible luminescence of chemically synthesized ZnS:Co film phosphors

Undoped and Cobalt doped ZnS thin films have been synthesised using chemical bath deposition technique. The X-ray diffraction pattern revealed a hexagonal structure for all the films. An increase in Co/Zn molar ratio resulted in a decrease in the intensity of diffraction peak corresponding to (100) plane and increase in crystallite size of the samples. The transmittance of the samples in the visible region was found to improve on doping. The optical band gap was found to vary from 3.46 to 3.66 eV with the incorporation of cobalt ions. The scanning electron microscope images of the samples exhibit a denser and more compact morphology for the doped films as compared to the undoped film. Photoluminescence studies reveal that all samples exhibit rare excitonic or near band-edge luminescence along with emissions in the visible region. The luminescence efficiency of ZnS film is appreciably enhanced with increase in concentration of the dopant.     

Structural,morphological, optical and gas sensing properties of pure and Ce doped SnO<Subscript>2</Subscript> thin films prepared by jet nebulizer spray pyrolysis (JNSP) technique

Pure and cerium (Ce) doped tin oxide (SnO₂) thin films are prepared on glass substrates by jet nebulizer spray pyrolysis technique at 450 °C. The synthesized films are characterized by X-ray diffraction (XRD), scanning electron microscopy, energy dispersive analysis X-ray, ultra violet visible spectrometer (UV–Vis) and stylus profilometer. Crystalline structure, crystallite size, lattice parameters, texture coefficient and stacking fault of the SnO₂ thin films have been determined using X-ray diffractometer. The XRD results indicate that the films are grown with (110) plane preferred orientation. The surface morphology, elemental analysis and film thickness of the SnO₂ films are analyzed and discussed. Optical band gap energy are calculated with transmittance data obtained from UV–Visible spectra. Optical characterization reveals that the band gap energy is found decreased from 3.49 to 2.68 eV. Pure and Ce doped SnO₂ thin film gas sensors are fabricated and their gas sensing properties are tested for various gases maintained at different temperature between 150 and 250 °C. The 10 wt% Ce doped SnO₂ sensor shows good selectivity towards ethanol (at operating temperature 250 °C). The influence of Ce concentration and operating temperature on the sensor performance is discussed. The better sensing ability for ethanol is observed compared with methanol, acetone, ammonia, and 2-methoxy ethanol gases.     

Growth of CZTS thin films by sulfurization of sputtered single-layered Cu–Zn–Sn metallic precursors from an alloy target

Cu₂ZnSnS₄ (CZTS) thin films were prepared by sulfurizing single-layered metallic Cu–Zn–Sn precursors which were deposited by DC magnetron sputtering using a Cu–Zn–Sn ternary alloy target. The composition, microstructure and properties of the CZTS thin films prepared under different sputtering pressure and DC power were investigated. The results showed that the sputtering rate of Cu atom increases as the sputtering pressure and DC power increased. The microstructure of CZTS thin films can be optimized by sputtering pressure and DC power. The CZTS thin film prepared under 1 Pa and 30 W showed a pure Kesterite phase and a dense micro-structure. The direct optical band gap of this CZTS thin film was calculated as 1.49 eV with a high optical absorption coefficient over 10⁴ cm^?1. The Hall measurement showed the film is a p-type semiconductor with a resistivity of 1.06 Ω cm, a carrier concentration of 7.904 × 10¹⁷ cm^?3 and a mobility of 7.47 cm² Vs^?1.     

Synthesis and characterization of Sn1Mg1−xO2 thin films fabricated by aero-sole assisted chemical vapor deposition

In order to achieve high conductivity and transmittance of transparent conducting oxide, Mg doped SnO₂ (Mg_xSn_1?xO₂) thin films have been fabricated and characterized to investigate their structural and optical properties. The Mg_xSn_1?xO₂ thin films have been deposited on glass substrate using aero-sole assisted chemical vapor deposition. The molar concentration of Mg contents was changed from 0 to 8 %. The confirmation of tetragonal structure and particle size (32–87 nm) of thin films was analyzed by X-ray diffraction. The surface roughness has been found to decrease with the increase of the dopant concentration as investigated by atomic force microscopy. The optical transmission increased from 54 to 78 % and the band gap of pure SnO₂ has been found to be 3.75 eV while it rises up to 3.88 eV with increasing Mg doping. The sheet resistance (R_s) of undoped SnO₂ is maximum which become lowest at 4 % Mg doped SnO₂.